The present invention relates to a process for rapidly terminating the polymerisation of vinyl chloride in aqueous suspension when polymerisation is carried out in the presence of lower dialkyl peroxydicarbonates.
Lower dialkyl peroxydicarbonates are widely used and highly active initiators for the polymerisation of vinyl chloride in aqueous suspension. Because of the exothermic nature of the polymerisation reaction it is advisable to take precautions to keep the reaction effectively under control. In particular the equipment used for this purpose provides for adequate stirring and cooling of the polymerisation medium.
The occurrence of an unforeseen defect in one or more of the control means during polymerisation leads to a runaway reaction which, due to the resulting uncontrollable increase in the temperature of the reaction medium, results at best in degradation of the polymer. If the runaway polymerisation cannot be rapidly brought under control far more serious trouble may result such as rupture of the bursting disc on the autoclave and the loss of monomer and polymer.
For reasons of safety and economy it is therefore essential for effective means to be available for rapidly terminating the polymerisation of vinyl chloride in aqueous suspension in the presence of lower dialkyl peroxydicarbonates should this prove to be necessary. The larger the capacity of the polymerisation reactor the more essential it is for such means to be provided.
It is generally known that polymerisation in the heterogeneous phase can be terminated by adding inhibitors consisting of inorganic reducing agents such as sodium bisulphite and dithionite, hydroxylamine or hydrazine to the polymerisation medium when the desired conversion has been achieved (Houben Weyl, Methoden der Organischen Chemie, Makromolekulare Stoffe, Part 1, pp. 434-435).
It has also been proposed to deactivate lower dialkyl peroxydicarbonates used in the polymerisation of vinyl chloride in aqueous suspension by adding basic substances such as alkali and alkaline earth metal and ammonium hydroxides, carbonates and bicarbonates to the polymerisation medium (Belgian Patent 846 908 filed on 4 October 1976 in the name of SOLVAY & Cie).
However, the prior art deactivators and inhibitors of the reducing agent type have proved to be insufficiently active for effectively and rapidly terminating the polymerisation of vinyl chloride in aqueous suspension in the presence of lower dialkyl peroxydicarbonates, particularly when conversion is still low.